1. Field of the Invention
This invention relates to a rectifier regulator system adapted to receiving power from a source of AC power such as a permanent magnet or rotating field alternator and supplying direct current to a load such as the communications and navigation radios in an aircraft. This regulator may take the form of a two stage device, the first stage being a switching regulator, the second stage being a linear device in the form shown. These are adapted to work together, however each stage defines unique improvements over the prior art in their own field.
2. Description of Prior Art
Typical small aircraft power systems of recent manufacture contain an adaptation of an automotive-type alternator in which a rotating member consists of steel poles and a copper winding to produce the required rotating magnetic field. The power to this winding is supplied through slip rings or similar devices. The stator which is stationary, usually is wound three phase and connected through diodes to the output terminal of the alternator which is connected to the aircraft battery. A solid state regulator senses the charging voltage of the battery and controls the excitation current to the field either by changing the effective resistance value between the field and the battery or by switching the connection between the battery and the field on and off at a high rate with an on duty cycle depending on the field current required to maintain the required output.
An alternator regulator setup of this type produces large amounts of electrical noise arising from arcing of the slip rings, switching transients in the field circuit and from recovery time transients associated with the diodes in the stator output circuit. It is general practice to use capacitive, or a combination inductive and capacitive filtering, in the output of the alternator to somewhat reduce this noise level. Also the impedance of the battery in parallel with the output bypasses much of this noise to ground. However, even with these precautions the remaining noise level is enough to be noticeable and therefore degrades the performance level of modern avionics. The response time of this type of regulator is long, compared to the period of the AC output of the alternator winding. Therefore this type of system cannot reasonably operate without a battery in the system. Removal of the battery, either intentionally to save weight in certain applications or inadvertently because of failure of the battery or associated wiring often results in overvoltage failures of avionics.
It should be noted it is common practice to turn off all radios in an aircraft before starting or stopping engines because of the transients which may be created by the alternators as well as other systems are of such magnitude they may be destructive to avionics. Further, the brushes and slip rings associated with this system have limited life, particularly at high altitude. Mean/time before brush failure of some alternators used in general aviation aircraft today, when operated at 20,000 feet, is below 50 hours. Further, the residual magnetism in these alternators is often so low that if the engine is started, such as by hand propping, when the battery is completely discharged, the alternator will not start to charge the battery.
Permanent magnet alternators can be manufactured by currently known processes to produce approximately the same output for approximately the same weight and speed as the wound rotor machines; however, the permanent magnet alternators have generally not found use in such general aviation airborne applications. One of the reasons is the lack of a suitable regulator. Alternators of this type are often employed on such vehicles as motorcycles, snowmobiles, and outboard motors. In these cases, regulation is accomplished generally by use of switching devices such as silicon controlled rectifiers, either in series or in shunt, as part of the network between the alternator and the battery. This type of rectifier-regulator combination produces high values of electromagnetic interference commonly called radio noise, which is generally not a problem in the application previously mentioned because these vehicles do not commonly have electronics susceptible to such noise as aircraft must for communications and navigations.
The objectives and purposes of this invention are as follows:
(1) to produce a simple, light, inexpensive, and highly efficient system to rectify and regulate the output of an alternator and to produce pure DC suitable for operating the electrical systems of an aircraft or other vehicle, so that this output may be compatible with the needs of solid state electronics devices such as communication and navigation radios as well as computer and guidance systems of the type found in airborne applications;
(2) to eliminate the need for a battery completely or to permit the continuing operation of the electronics of a system with a battery in case of battery failure;
(3) to eliminate any transients upon the starting and stopping of the alternator, rectifier, regulator system which may damage the loads connected to the system;
(4) to produce a highly accurate output voltage unaffected by wide ranges of load or speed of the alternator or by a wide range of temperature;
(5) to produce a power system which will not be damaged by sudden changes in load to include all load resistances from zero to infinity and that even with sudden steps in magnitude of the load will not produce transients or overshoots in the output which might damage solid state devices contained in the loads;
(6) to produce a power system in which the power semiconductor devices are all attached electrically as well as thermally to the heat dissipating surfaces either of a grounded heat sink or one additional isolated heat sink;
(7) to produce a power system capable of starting without a battery or any other source of external power, under even the adverse combination of very low ambient temperature such as -65.degree. C. and a load resistance corresponding to several times lower than the resistance which would normally be associated with the full power output of the alternator. This last stated purpose is particularly desirable since certain types of airborne equipment such as the power supply sections of increasingly common radar systems present a negative impedance to the power system that supply their direct current input power; and because aircraft are frequently required to operate at very low ambient temperatures.